The phc2sys program is used to synchronize the system clock to the PTP hardware clock (PHC) on the NIC. To start phc2sys, where em3 is the interface with the PTP hardware clock, enter the following command as root:

~]# phc2sys -s em3 -w

The -w option waits for the running ptp4l application to synchronize the PTP clock and then retrieves the TAI to UTC offset from ptp4l.

Normally, PTP operates in the International Atomic Time (TAI) timescale, while the system clock is kept in Coordinated Universal Time (UTC). The current offset between the TAI and UTC timescales is 35 seconds. The offset changes when leap seconds are inserted or deleted, which typically happens every few years. The -O option needs to be used to set this offset manually when the -w is not used, as follows:

~]# phc2sys -s em3 -O -35

Once the phc2sys servo is in a locked state, the clock will not be stepped, unless the -S option is used. This means that the phc2sys program should be started after the ptp4l program has synchronized the PTP hardware clock. However, with -w, it is not necessary to start phc2sys after ptp4l as it will wait for it to synchronize the clock.

The phc2sys program can also be started as a service by running:

~]# systemctl start phc2sys

When running as a service, options are specified in the /etc/sysconfig/phc2sys file. More information on the different phc2sys options can be found in the phc2sys(8) man page.

Note that the examples in this section assume the command is run on a slave system or slave port.